Introduction: The Asteroid Impact Deflection Assessment (AIDA) mission will be the first space experiment to demonstrate asteroid impact hazard mitigation by using a kinetic impactor. AIDA is a joint ESA-NASA cooperative project, consisting of the NASA Double Asteroid Redirection Test (DART) kinetic impactor mission [1] and the ESA Asteroid Impact Mission (AIM), which is the asteroid rendezvous spacecraft [2]. The original AIM concept did not receive full funding in late 2016, but a rescoping of AIM is being undertaken at ESA. The AIDA target is the near-Earth binary asteroid 65803 Didymos. During the Didymos close approach to Earth in October, 2022, the DART spacecraft will impact the Didymos secondary at 6 km/s and deflect its trajectory, changing the orbital period of the binary. This change can be measured by Earth-based optical and radar observations. The primary goals of AIDA are to (1) perform a full-scale demonstration of asteroid deflection by kinetic impact; (2) measure the resulting deflection; and (3) validate and improve models for momentum transfer in high-speed impacts on an asteroid. The combined DART and AIM missions will provide the first measurements of momentum transfer efficiency from a kinetic impact at full scale on an asteroid, where the impact conditions of the projectile are known, and physical properties and internal structures of the target asteroid are also characterized. AIDA with both DART and AIM will be the first fully documented impact experiment at asteroid scale, including characterization of the target’s properties and the outcome of the impact to test and refine our understanding and models at an actual asteroid scale. AIDA will check whether current extrapolations of material strength from laboratory scale to asteroid scale are valid. AIDA will validate the kinetic impactor technique to deflect a small body and reduce risks for future asteroid hazard mitigation. DART: The momentum transfer efficiency β of a kinetic impactor is the ratio of the momentum transferred to the target over the incident momentum. Because there is momentum carried away by impact ejecta released back towards the incident direction, this β generally exceeds unity [1,3,4]. There are many unknowns that affect β, which is critical to predicting the amount of deflection to be achieved by a kinetic impact. By performing a kinetic impact on an asteroid and observing the target both before and after the impact, AIDA will measure β and determine the magnitude and direction of deflection. It will measure physical properties of the target asteroid, especially density, and determine shape and geology of the impact site. It will further study outcomes such as changes in the target body rotation state and evolution of ejecta. Mission and Payload: The DART kinetic impactor baseline mission has changed from that given in [1]. DART will launch as a secondary payload to geosynchronous orbit and use the NASA Evolutionary Xenon Thruster (NEXT) ion propulsion system to spiral out from Earth orbit and transfer to Didymos (see Table 1). For a launch on or before 31 March 2021, the Didymos impact will now occur on Oct. 7, 2022, a few weeks later than in [1]. With a larger ~490 kg spacecraft impacting at 6 km/s, the incident momentum is significantly increased from that in [1], leading to a larger target deflection and a larger crater.